Thesis defense [before December 2013]

Licentiate thesis: Magnetic Helicity Fluxes and their Effects in Dynamo Theory

by Simon Candelaresi (Nordita)

FB42 ()


The roles of magnetic helicity and magnetic helicity fluxes in astrophysical objects are investigated using various models and field configurations. Their roles in dynamo theory are confirmed through magnetohydrodynamic simulations both within the framework of mean-field theory and in direct numerical simulations.
The constraint of magnetic helicity conservation in a periodic system at high magnetic Reynolds numbers is analyzed for setups of three magnetic flux rings which can be interlocked. The linking is able to hinder the magnetic field to decay only if the linking implies magnetic helicity. If the magnetic field is not helical the decay shows the same behavior irrespective of the actual linking of the rings which supports the assumption that only the magnetic helicity is the decisive topological quantity in magnetic relaxation.
The regime of high magnetic Reynolds numbers is analyzed by using a one-dimensional mean-field model for a helically forced dynamo. A wind with linear profile is imposed such that magnetic helicity can be advected to one of the domain boundaries. It is shown that with vacuum boundary conditions helicity can be shed from of the domain, which alleviates the quenching at high magnetic Reynolds numbers. Additionally the same boundary is closed for a different setup where a diffusive flux is allowed at the midplane of the system. This is shown to also reduce the quenching mechanism and to allow for dynamo action at large magnetic Reynolds numbers.
The influence of the gauge on magnetic helicity transport and fluxes is explored in the Weyl gauge, the resistive gauge and the pseudo-Lorenz gauge as well as a newly introduced advecto-resistive gauge. In the first three gauges spatially averaged fluxes are analyzed and compared with the one-dimensional mean-field model. The alleviation of the quenching is independent of the gauge as it was expected since it is a physical effect. In the advecto-resistive gauge magnetic helicity density evolves like a passive scalar in the kinematic regime owing it to the advective nature of the gauge. In the dynamical regime magnetic helicity is advected into length scales of the turbulent eddies.

Licentiate Thesis